Cleanup test code for symbolic links.

Change-Id: I7a116e4d5c0e955578eca53c1af559e9092f60cd
Reviewed-on: https://go-review.googlesource.com/27572Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>